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I found an interesting and confusing phenomenon when I was coding, and I did some test. Here is the test code.

testlist = [[]]*2
for i in range(2):
    for j in range(11):
        if j%2==i:
            testlist[i].append(j)
print(testlist)

I thought the output should be

[[0, 2, 4, 6, 8, 10], [ 1, 3, 5, 7, 9]]

but the output actually is

[[0, 2, 4, 6, 8, 10, 1, 3, 5, 7, 9], [0, 2, 4, 6, 8, 10, 1, 3, 5, 7, 9]]

And I changed my code a little bit

testlist = [[]]*2
for i in range(2):
    for j in range(11):
        if j%2==i:
            testlist[0].append(j)
print(testlist)

note that only testlist[0] is mentioned so testlist[1] shouldn't be changed at all, but the output is still

[[0, 2, 4, 6, 8, 10, 1, 3, 5, 7, 9], [0, 2, 4, 6, 8, 10, 1, 3, 5, 7, 9]]

This confuses me so much and a possible explanation I come up with is that by using [[]]*2, testlist[0] and testlist[1] are pointing to the same object. Because the following code runs well.

testlist = [[],[]]
for i in range(2):
    for j in range(11):
        if j%2==i:
            testlist[i].append(j)
print(testlist)

[[0, 2, 4, 6, 8, 10], [1, 3, 5, 7, 9]]

Can anybody give me a solid explanation? And If I want to make a list of 1000 empty lists, how can I do that without typing 1000 []s?

FacelessMage
  • 1
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0 Answers0